Vegetated roofs are filters that treat pollutants in precipitation and are of two general types: green roofs (Figure 1) and ecoroofs (Figure 2), a name less commonly used. Each consists of a building roof that is partially or completely covered with vegetation. The species are selected optimally for the particular environment, and a thin substrate is placed over a waterproof membrane. The structure includes a root barrier and a drainage system (Figure 3). Vegetated roofs serve several purposes, such as absorbing rainwater, providing insulation, helping to lower urban air temperatures, and combating the effect of heat islands. Most if not all of the captured rainwater is lost to the atmosphere by evapotranspiration (Figure 4). The roofs are sloped to assist drainage. A vegetated roof is in effect a thin bioretention system. Green roofs are typically fertilized, often being in view of the public, whereas ecoroofs are not–hence the prefix eco.
Types
Vegetated roofs can be categorized as intensive, semi-intensive, or extensive, depending on the depth of planting medium and the amount of maintenance each needs. Figure 5 lists characteristics of the two extreme structures: extensive and intensive. Traditional roof gardens, which require a reasonable depth of soil to grow large plants or conventional lawns, are considered intensive because they are labor-intensive, requiring irrigation, fertilizer, and other maintenance. Intensive roofs are more park-like with easy access and may include anything from kitchen herbs to shrubs and small trees.
Extensive roofs, by contrast, are designed to be virtually self-sustaining (Figure 4). They
require minimal maintenance, perhaps a once-yearly weeding, and less frequently an application of slow-release fertilizer to boost growth. Extensive roofs are usually only accessed for maintenance. They can be established on a very thin layer of “soil” (most use specially formulated composts). Even a thin layer of rock wool laid directly onto a watertight roof can support a planting of sedum species and mosses. These are the ecoroofs.
Disadvantages
The main disadvantage of vegetated roofs is their higher initial cost compared to conventional roofs. Waterproofing systems and root barriers increase the initial cost. Intensive green roofs involve some maintenance cost, but the other two types generally do not. Regardless, life-cycle costing will show a lower long-term cost than that of a standard roof. This is due to the vegetated roof protecting the membranes from the elements, particularly ultraviolet radiation of the sun. The life expectancy of the membranes is doubled or even tripled in comparison to conventional roofs.
Some types of vegetated roofs will have more demanding structural standards in seismic regions. Some existing buildings cannot be retrofitted with an intensive vegetated roof because the weight of the substrate and the vegetation exceeds permitted static loading.
The intensive vegetated roof will require irrigation in dryer environments, which adds weight that may not be tolerated in certain retrofit situations. In contrast, a sedum covering doesn’t need irrigation, as these plants tolerate long periods of dryness.
Hydrologic Performance
Vegetated roofs are primarily a hydrologic system, reducing volumes and rates to predevelopment levels. Vegetated roofs are capable of removing 50% or more of the annual rainfall volume through evapotranspiration. Rainfall not retained is detained, increasing the time to peak and slowing peak flows to more closely mimic natural hydrology. In temperate climates more runoff is generated during winter, and for many summer storms there is no runoff. There are methods to evaluate the hydrologic benefits. Vegetated roofs significantly reduce runoff volumes (40 to 90%) and peak rates (80 to 95%). The effect on volume retained is seasonal, with a greater percentage being retained in the dry months.
One study over 18 months found that runoff in April and November averaged 42% less from a vegetated roof than that from a conventional roof studied, and 70 to 93% less in the intervening months of May through October. The roof, located in Toronto, was not studied during the winter. Another study compared intensive to extensive vegetated roofs. The obvious was determined: that the extensive roof contains more water and reduces the outflow of water to a greater extent than do intensive roofs, and that the amount of water detained depends on the season. Roof thickness affects the discharge ratio, which is the ratio of how much volume is discharged to how much comes into the roof.
Vegetated roofs have evolved from any planted roof to those that tend to be well maintained for visual appearance because they are within view of the public. The name ecoroof is most often applied to vegetated roofs that are not in public view and whose appearance is therefore unimportant. That said, one might presume that ecoroofs provide better water quality because of less fertilizer use. What is aesthetic of course depends upon the region, given the differences in expectations as to what a vegetated roof should look like.
Treatment Performance
The quality of stormwater from conventional roofs varies tremendously with the roof type. The water quality of runoff from vegetated roofs is likely more consistent, with the major variations between the two types of vegetated roofs. Many studies have been conducted on vegetated roofs, sometimes comparing the results to conventional roofs.
Vegetated roofs have been found to leach nutrients, given the need for fertilizers that are used for aesthetic reasons. Although precipitation contains a substantial amount of nitrogen (2 to 3 gms/m2/year), it is insufficient for growth (5 gms/m2/year) for extensive roofs. It is sufficient, however, for intensive roofs. They will reduce the concentration of some pollutants in rainwater, such as mercury. Nitrogen is transformed from ammonia in the precipitation to nitrate in the runoff, the transformation occurring in the roof soils. Copper concentration was found to increase over precipitation. Zinc was lower from a vegetated roof in comparison to asphalt roofs. Guidelines are available on monitoring.
One study of extensive roofs found that with coated slow-release fertilizers, the release of nutrients was gradual and reasonable. Sixteen percent of the nitrogen was lost at an application rate of 5 gms/m2/year. The authors noted that there have been few studies of intensive roofs. The information presented here pertains to extensive roofs.
Nitrogen and phosphorus concentrations were found to be higher in runoff from a vegetated roof in comparison to precipitation and to runoff from a conventional roof. Loading release was also higher. Another study found that the concentrations of phosphorus exceeded the receiving water standard.
Compost, if used in the growing medium, may be a source of nutrients. The nitrate leaches from the roof. However, loss of nutrients likely decreases within a few years absent additional fertilization.
Some vegetated roofs have been found to release most metals whereas others have not. This may relate to differences in composts. One study found significant retention for two metals: 95% for lead and 88% for cadmium. Another study observed stormwater that was brown, with a higher biological oxygen demand (BOD) than typical stormwater. A study also found increases in bacteria counts, no doubt due to birds.
Pesticide leakage has been observed from vegetated roofs. Care must be taken in the selection of thick membranes, the apparent source of the pesticides. It has also been observed that vegetated roof materials that leak carbonates should be avoided.
Pollutant concentrations can be higher due to evapotranspiration. Hence, loading is more relevant than concentration analysis. Due to the volume reduction, pollutant loadings are less than those in runoff from asphalt, or are otherwise manageable by downspout treatment, with exceptions such as phosphorous and nitrogen. However, the loadings in the runoff as compared to loadings from the precipitation were found to be the same in one study but higher in a second study. Vegetated roofs are of particular value in cold climates. They also reduce the heat island effect of dense urban cores.